Nowadays it is common that a home or business electronic appliance is equipped with an EMI filter at the power input port so as to suppress the EMI, to prevent the conducted electromagnetic disturbances generated by this appliance from interfering with other electronic devices nearby, and to prevent conducted electromagnetic disturbances generated by the electronic devices nearby from interfering with the operation of the appliance. The design goal of an EMI filter includes suppressing the voltages corresponding to the EMI within a certain spectrum to meet various international EMC (electromagnetic compatibility) standards so that the product can pass the EMC certification tests.
FIG. 1 is a circuit diagram of a simple EMI filter according to a related art. Referring to FIG. 1, the EMI filter includes a common mode choke LCM, a resistor R, and capacitors CX, CY and CY′.
FIG. 2 is a circuit diagram of a relatively complete EMI filter according to a related art. Referring to FIG. 2, the EMI filter includes a differential mode choke LDM, a common mode choke LCM, a resistor R, and capacitors CX1, CX2, CY and CY′. When the interference voltages are relatively high, to make the product pass the certification tests, in addition to increasing the common mode inductance to suppress the common mode interference voltages, the differential mode choke is included in the EMI filter network, which has a clear effect on the suppression of the differential mode interference voltages.
However, including the differential mode choke into the EMI filter network will lead to the increase in volume, weight, cost and power loss of the EMI filter. Because of the wide usage and the great number of EMI filters, it is desirable to decrease the volume, weight, cost and power consumption of EMI filters.